Light-emitting element, display device, and electronic appliance

ABSTRACT

The present invention provides a light-emitting element having a structure in which the drive voltage is comparatively low and a light-emitting element in which the increase in the drive voltage over time is small. Further, the present invention provides a display device in which the drive voltage and the increase in the drive voltage over time are small and which can resist long-term use. A layer in contact with an electrode in a light-emitting element is a layer containing a P-type semiconductor or a hole-generating layer such as an organic compound layer containing a material having electron-accepting properties. The light-emitting layer is sandwiched between the hole-generating layers, and an electron-generating layer is sandwiched between the light-emitting layer and the hole-generating layer on a cathode side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element whichsandwiches a thin film containing a light-emitting material betweenelectrodes and which emits light by applying current. Moreover, thepresent invention relates to a display device and an electronicappliance which use the light-emitting element.

2. Related Art

A display using a thin film light-emitting element of aself-light-emitting type, which emits light by itself when current isapplied, has been extensively developed.

This thin film light-emitting element emits light by connecting anelectrode to a single-layer or multilayer thin film formed using one orboth of organic compound and inorganic compound and by applying current.Such a thin film light-emitting element is expected to reduce the powerconsumption, occupy smaller space, and increase the visibility, and themarket is also expected to expand further.

It has become possible to manufacture an element which emits light moreefficiently than before by dividing the function for each layer of alight-emitting element having a multilayer structure (for example, seeReference 1: Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987) byC. W. Tang et al.).

A thin film light-emitting element having a multilayer structure has alight-emitting laminated body sandwiched between an anode and a cathode.The light-emitting laminated body comprises a hole-injecting layer, ahole-transporting layer, a light-emitting layer, anelectron-transporting layer, an electron-injecting layer, and the like.Among these layers, the hole-injecting layer, the hole-transportinglayer, the electron-transporting layer, and the electron-injecting layerare not always employed depending on the element structure.

The hole-injecting layer in the light-emitting laminated body as aboveis formed by selecting a material which can inject holes comparativelyeasily from a metal electrode into a layer mainly containing organiccompound. The electron-transporting layer in the light-emittinglaminated body is formed by selecting a material being superior inelectron-transporting properties. Thus, each layer in the light-emittinglaminated body is formed by selecting a material superior in eachfunction.

However, a material which can inject electrons comparatively easily froman electrode into a material mainly containing organic compound, or amaterial mainly containing organic compound which can transportelectrons at a predetermined mobility or more are very limited. As isclear from the limitation on the material, the injection of theelectrons from the electrode into the layer mainly containing theorganic compound is originally rare to occur. This causes the problemthat the drive voltage increases drastically over time.

SUMMARY OF THE INVENTION

Consequently, it is an object of the present invention to provide alight-emitting element having a structure in which the increase in thedrive voltage over time is small.

Further, it is an object of the present invention to provide a displaydevice in which the drive voltage is low and the increase in the drivevoltage over time is small and which can resist long-term use.

According to the present invention, a layer in contact with an electrodein a light-emitting element is a hole-generating layer such as a layercontaining a P-type semiconductor or an organic compound layercontaining a material having electron-accepting properties, alight-emitting layer is sandwiched between the hole-generating layers,and an electron-generating layer is formed between the hole-generatinglayer on a cathode side and the light-emitting layer. This enables theincrease in the drive voltage over time to be small.

A light-emitting element having one of structures according to thepresent invention comprises a pair of electrodes including an anode anda cathode, a first layer and a second layer for generating holes, athird layer containing a light-emitting material, and a fourth layer forgenerating electrons, wherein the third layer is sandwiched between thefirst layer and the second layer which are provided between theelectrodes, wherein the fourth layer is provided between the third layerand the second layer, and wherein the second layer contacts the cathode.

A light-emitting element having one of structures according to thepresent invention comprises a pair of electrodes including an anode anda cathode, a first layer and a second layer which contain a P-typesemiconductor, a third layer containing a light-emitting material, and afourth layer containing an N-type semiconductor, wherein the third layeris sandwiched between the first layer and the second layer which areprovided between the electrodes, wherein the fourth layer is providedbetween the third layer and the second layer, and wherein the secondlayer contacts the cathode.

A light-emitting element having another structure according to thepresent invention comprises a pair of electrodes including an anode anda cathode, a first layer and a second layer which contain a firstorganic compound and a material having electron-accepting properties tothe first organic compound, a third layer which contains alight-emitting material, and a fourth layer which contains a secondorganic compound and a material having electron-donating properties tothe second organic compound, wherein the third layer is sandwichedbetween the first layer and the second layer which are provided betweenthe electrodes, wherein the fourth layer is provided between the thirdlayer and the second layer, and wherein the second layer contacts thecathode.

In a light-emitting element having a structure according to the presentinvention, the increase in the drive voltage over time can besuppressed.

Further, a display device in which the increase in the drive voltageover time is small and which can resist long-term use can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 shows a light-emitting element according to the presentinvention;

FIG. 2 shows a light-emitting element according to the presentinvention;

FIG. 3 shows a light-emitting element according to the presentinvention;

FIG. 4 shows a light-emitting element according to the presentinvention;

FIGS. 5A to 5E show a process for manufacturing a thin filmlight-emitting element according to the present invention;

FIGS. 6A to 6C show a process for manufacturing a thin filmlight-emitting element according to the present invention;

FIGS. 7A and 7B show an example of a structure of a display deviceaccording to the present invention;

FIGS. 8A and 8B are a top view and a cross-sectional view of alight-emitting device according to the present invention;

FIGS. 9A to 9E show examples of electronic appliances to which thepresent invention can be applied;

FIGS. 10A to 10C show examples of a structure of a display deviceaccording to the present invention;

FIGS. 11A to 11F show examples of a pixel circuit of a display deviceaccording to the present invention;

FIG. 12 shows an example of a protective circuit of a display deviceaccording to the present invention;

FIG. 13 is a graph showing the voltage-luminance characteristic of anelement in Embodiment 1;

FIG. 14 is a graph showing the voltage-current characteristic of anelement in Embodiment 1;

FIG. 15 is a graph showing the voltage-luminance characteristic of anelement in Embodiment 2;

FIG. 16 is a graph showing the current density-luminance characteristicof an element in Embodiment 2;

FIG. 17 is a graph showing the voltage-current characteristic of anelement in Embodiment 2;

FIG. 18 is a graph showing the change of voltage of an element over timein Embodiment 2;

FIG. 19 is a graph showing the change of luminance of an element overtime in Embodiment 2;

FIG. 20 is a graph showing the voltage-luminance characteristic of anelement in Embodiment 3;

FIG. 21 is a graph showing the voltage-current characteristic of anelement in Embodiment 3;

FIGS. 22A to 22C show absorption spectra of a complex materialcontaining α-NPD and molybdenum oxide;

FIGS. 23A to 23C show absorption spectra of a complex materialcontaining DNTPD and molybdenum oxide;

FIG. 24 is a graph showing a relation between an optical distance andcurrent efficiency; and

FIG. 25 is a graph showing an optical distance and a light-emissionspectrum.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment Modes and Embodiments are hereinafter described withreference to the drawings. However, since the present invention can becarried out with many different modes, it is to be understood by thoseskilled in the art that the modes and details can be modified withoutdeparting from the scope of the present invention. Therefore, thepresent invention is not construed as being limited to the descriptionof the following Embodiment Modes and Embodiments.

Embodiment Mode 1

The present embodiment mode describes the structure of a light-emittingelement of the present invention with reference to FIGS. 1 and 2. In alight-emitting element according to the present invention, alight-emitting layer 104 containing a light-emitting material and anelectron-generating layer 105 are stacked, and the light-emitting layer104 and the electron-generating layer 105 are sandwiched between a firsthole-generating layer 102 and a second hole-generating layer 103. Thefirst hole-generating layer 102 and the second hole-generating layer 103are further sandwiched between an anode 101 and a cathode 106, andstacked over an insulator 100 such as a substrate or an insulating film.Over the insulator 100 such as the substrate or the insulating film, theanode 101, the first hole-generating layer 102, the light-emitting layer104, the electron-generating layer 105, the second hole-generating layer103, and the cathode 106 are stacked in order (FIG. 1). Alternatively,the order may be opposite: the cathode 106, the second hole-generatinglayer 103, the electron-generating layer 105, the light-emitting layer104, the first hole-generating layer 102, and the anode 101 are stackedin order (FIG. 2).

The first hole-generating layer 102 and the second hole-generating layer103 may be formed either with different materials or with the samematerial. For example, a layer containing both of a hole-transportingmaterial and an electron-accepting material which can receive electronsfrom the hole-transporting material, a P-type semiconductor layer, or alayer containing a P-type semiconductor is used. As thehole-transporting material, for example, an aromatic amine compound(having a bond of a benzene ring with nitrogen), phthalocyanine(abbreviated to H₂Pc), or a phthalocyanine compound such as copperphthalocyanine (abbreviated to CuPc) or vanadyl phthalocyanine(abbreviated to VOPc) can be used. The aromatic amine compound is, forexample, 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviatedto α-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl(abbreviated to TPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine(abbreviated to TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviated to MTDATA),4,4′-bis(N-(4-(N,N-di-m-tolylamino)phenyl)-N-phenylamino)biphenyl(abbreviated to DNTPD), 1,3,5-tris[N,N-di(m-tolyl)amino]benzene(abbreviated to m-MTDAB), or 4,4′,4″-tris(N-carbazolyl)triphenylamine(abbreviated to TCTA). As the electron-accepting material which canreceive electrons from the hole-transporting material, for example,vanadium oxide, molybdenum oxide, 7,7,8,8,-tetracyanoquinodimethane(abbreviated to TCNQ), 2,3-dicyanonaphtoquinone (abbreviated to DCNNQ),2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (abbreviated toF₄-TCNQ), or the like is given. The electron-accepting material isselected which can receive electrons in accordance with the combinationwith the hole-transporting material. Further, metal oxide such asmolybdenum oxide, vanadium oxide, ruthenium oxide, cobalt oxide, nickeloxide, or copper oxide can be used as the P-type semiconductor.

As the electron-generating layer 105, a layer containing both of anelectron-transporting material and an electron donating material whichcan donate electrons to the electron-transporting material, an N-typesemiconductor layer, or a layer containing an N-type semiconductor canbe used. As the electron-transporting material, for example, thefollowing can be used; a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as tris-(8-quinolinolato)aluminum(abbreviated to Alq₃), tris(4-methyl-8-quinolinolato)aluminum(abbreviated to Almq₃), bis(10-hydroxybenzo[h]-quinolinolato)beryllium(abbreviated to BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq). Besides, a metal complex having an oxazole or thiazole ligandsuch as bis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated toZn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolate]zinc (abbreviated toZn(BTZ)₂) can be used. In addition to the metal complex,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated toPBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated to OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to p-EtTAZ), bathophenanthroline (abbreviated to BPhen),bathocuproin (abbreviated to BCP), or the like can be used. As theelectron donating material which can donate electrons to theelectron-transporting material, for example, alkali metal such aslithium or cesium, magnesium, alkali-earth metal such as calcium, orrare-earth metal such as erbium or ytterbium can be used. The electrondonating material which can donate electrons is selected in accordancewith the combination with the electron-transporting material. Further, ametal compound such as metal oxide can be used as the N-typesemiconductor, and for example zinc oxide, zinc sulfide, zinc selenide,titanium oxide, or the like can be used.

The light-emitting layer 104 containing the light-emitting material isdivided into two types. One of them is a layer in which a light-emittingmaterial to be a luminescence center is diffused in a layer formed witha material having a wider energy gap than the light-emitting material.The other one is a layer consisting of a light-emitting material. Theformer structure is preferred because the concentration quenching isdifficult to occur. As the light-emitting material to be theluminescence center, the following can be employed;4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl)-4H-pyran(abbreviation: DCJT);4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]-4H-pyran;periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]benzene,N,N′-dimethylquinacridone (abbreviated to DMQd), coumarin 6, coumarin545T, tris(8-quinolinolato)aluminum (abbreviated to Alq₃),9,9′-bianthryl, 9,10-diphenylanthracene (abbreviated to DPA),9,10-bis(2-naphthyl)anthracene (abbreviated to DNA),2,5,8,11-tetra-t-butylperylene (abbreviated to TBP), or the like. As thematerial to be a base material in the case of forming the layer in whichthe light-emitting material is diffused, the following can be used; ananthracene derivative such as 9,10-di(2-naphtyl)-2-tert-butylanthracene(abbreviated to t-BuDNA), a carbazole derivative such as4,4′-bis(N-carbazolyl)biphenyl (abbreviated to CBP), or a metal complexsuch as tris(8-quinolinolato)aluminum (abbreviated to Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviated to Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviated to BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq), bis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviated to Znpp₂), orbis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated to ZnBOX). As thematerial which can constitute the light-emitting layer 104 singularly,tris(8-quinolinolato)aluminum (abbreviated to Alq₃),9,10-bis(2-naphtyl)anthracene (abbreviated to DNA), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq) or the like can be used.

The light-emitting layer 104 may be formed either in a single-layerstructure or a multilayer structure. A hole-transporting layer may beprovided between the first hole-generating layer 102 and the layer inwhich the light-emitting material is diffused in the light-emittinglayer 104. Further, an electron-transporting layer may be providedbetween the electron-generating layer 105 and the layer in which thelight-emitting material is diffused in the light-emitting layer 104.These layers are not necessarily provided. Alternatively, only one ofthe hole-transporting layer and the electron-transporting layer may beprovided. The materials of the hole-transporting layer and theelectron-transporting layer conform to those of the hole-transportinglayer in the hole-generating layer and the hole-transporting layer inthe electron-generating layer respectively; therefore, the descriptionis omitted here. Refer to the description of those layers.

The anode 101 is preferably formed with metal, alloy, an electricallyconductive compound each of which has high work function (work functionof 4.0 eV or more), or mixture of these. As a specific example of theanode material, the following can be used; ITO (indium tin oxide), ITOcontaining silicon, IZO (indium zinc oxide) in which zinc oxide (ZnO) ismixed by 2 to 20% into indium oxide, gold (Au), platinum (Pt), nickel(Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt(Co), copper (Cu), palladium (Pd), or metal nitride such as TiN.Meanwhile, as the cathode material used for forming the cathode 106, itis preferable to use metal, alloy, an electrically conductive compoundeach of which has low work function (work function of 3.8 eV or less),or mixture of these. As the specific example of the cathode material,the following can be used; an element belonging to group 1 or 2 in theperiodic table; alkali metal such as Li or Cs, Mg, or alkali-earth metalsuch as Ca or Sr. In addition, alloy containing the above element suchas Mg:Ag or Al:Li, a compound containing the above element such as LiF,CsF, or CaF₂, or transition metal containing rare-earth metal can alsobe used. Further, a multilayer containing the above element and anothermetal (including alloy) such as Al, Ag, or ITO can be used.

In addition to the anode 101, the first hole-generating layer 102, thelight-emitting layer 104, the electron-generating layer 105, the secondhole-generating layer 103, and the cathode 106, the light-emitting layermay have a hole-injecting layer 107 between the anode 101 and the firsthole-generating layer 102 (FIGS. 3 and 4). A phthalocyanine compound iseffective for the hole-injecting layer 107. For example, phthalocyanine(abbreviated to H₂—Pc), copper phthalocyanine (abbreviated to Cu—Pc), orthe like can be used.

The above material is just an example, and the material can be selectedappropriately by a practitioner as long as the advantage of the presentinvention is obtained.

In the light-emitting element having the above structure according tothe present invention, holes are injected from the secondhole-generating layer 103 into the second electrode by applying voltage.In addition, electrons are injected from the electron-generating layer105 into the light-emitting layer 104. Further, holes are injected fromthe first hole-generating layer 102 into the light-emitting layer 104.Then, the injected electrons and holes are recombined in thelight-emitting layer, and light emission is obtained when the excitedlight-emitting material returns to the ground state. Here, in thelight-emitting element according to the present invention, the electronsare not injected from the electrode into the layer mainly containing theorganic compound but injected from the layer mainly containing theorganic compound into the layer mainly containing the organic compound.The electrons are difficult to be injected from the electrode into thelayer mainly containing the organic compound. In the conventionallight-emitting element, the drive voltage has increased when theelectrons are injected from the electrode into the layer mainlycontaining the organic compound. However, since the light-emittingelement according to the present invention does not have such a process,the light-emitting element having low drive voltage can be provided.Moreover, it is already known from the experiment that the drive voltageincreases over time more drastically when the light-emitting element hashigher drive voltage; therefore, the light-emitting element having lowdrive voltage also serves as a light-emitting element in which theincrease in the drive voltage over time is small.

Embodiment Mode 2

Another embodiment mode of the present invention is described. Thepresent embodiment mode describes an example of improving thecharacteristic of a viewing angle of a light-emitting element and adisplay device by appropriately adjusting the thicknesses of the firsthole-generating layer 102 and the second hole-generating layer 103.Since the multilayer structure and the material of the light-emittingelement in the present embodiment mode are the same as those inEmbodiment Mode 1, the description is omitted here. Refer to EmbodimentMode 1.

Light emitted from the light-emitting element include light directlyemitted from the light-emitting layer 104 and light emitted after beingreflected once or multiple times. The light directly emitted and thelight emitted after being reflected interfere in accordance with therelation between their phases so that they are intensified or attenuatedwith each other. Therefore, the light emitted from the light-emittingelement is light which has been combined as a result of theinterference.

The phase of light reflected when entering a medium having highrefractive index from a medium having low refractive index is inverted.For this reason, in the light-emitting element having the structureshown in Embodiment Mode 1, the phase of light is inverted when thelight is reflected at the interface between the electrode such as theanode 101 or the cathode 106 and the layer in contact with theelectrode. When the light reflected at the electrode interferes with thelight emitted from the light-emitting layer, it is possible to decreasethe change of the spectrum shape which occurs depending on the angle ofviewing a surface from which light is extracted and to increase thecurrent efficiency of the light-emitting element, provided that theoptical distance (refractive index×physical distance) between thelight-emitting layer and the electrode satisfies (2m−1)λ/4 (m is anatural number of 1 or more and λ is a center wavelength of the lightemitted from the light-emitting layer). The current efficiency shows theluminance with respect to the flowed current. When the currentefficiency is higher, predetermined luminance can be obtained even witha smaller amount of current. Moreover, the deterioration of the elementtends to be little.

Since the reflection is small between films whose gap of refractiveindex is small, the reflections except the reflection at the interfacebetween the electrode and the film in contact with the electrode areignorable. Therefore, in this embodiment mode, attention is paid only tothe reflection between the electrode and the film in contact with theelectrode.

In the case of a light-emitting element in which light is extracted fromthe side of the anode 101, the light is reflected at the cathode 106.For this reason, in order to increase the current efficiency of thelight-emitting element and to decrease the change of the spectrum shapewhich occurs depending on the angle of viewing the surface from whichthe light is extracted, the optical distance (refractive index×physicaldistance) from the light-emission position to the surface of the cathode106 needs to be (2m−1)λ/4 (m is a natural number of 1 or more and λ is acenter wavelength of the light emitted from the light-emitting layer).

The light-emitting layer 104 may be formed in a single-layer structurewith a layer containing a light-emitting material, or may be formed in amultilayer structure including a layer such as an electron-transportinglayer or a hole-transporting layer and a layer containing alight-emitting material. The layer containing the light-emittingmaterial may be a layer in which a light-emitting material to be aluminescence center is diffused or may be a layer consisting of alight-emitting material.

A plurality of layers formed with different materials are providedbetween the light-emission position and the cathode 106. In thisembodiment mode, the plurality of layers correspond to theelectron-generating layer 105 and the second hole-generating layer 103.A part of the layer containing the light-emitting material thatcorresponds to a half thickness thereof can be regarded as a layerpositioned between the light-emission position and the cathode 106. Inthe case of forming the light-emitting layer with a plurality of layers,more layers formed with different materials may be included. In such astructure, the optical distance from the light-emission position to thecathode 106 can be calculated by multiplying the thicknesses and therefractive indexes of the respective films and summing up the products.The total is set so as to be (2m−1)λ/4 (m is a natural number of 1 ormore and λ is a center wavelength of the light emitted from thelight-emitting layer). That is to say, the following formula (1) issatisfied. In the formula (1), the layer containing the light-emittingmaterial is assumed to be 1 and the cathode 106 is assumed to be j (j isan integer number of 4 or more), and the layers existing between thelayer containing the light-emitting material and the cathode 106 aredenoted with numerals in order from the layer containing thelight-emitting material. Moreover, the refractive index n and thethickness d with a certain numeral given thereto indicate the refractiveindex and the thickness of the layer to which the same numeral is given(that is, n₁ is the refractive index of the layer containing thelight-emitting material and d_(j) is the thickness of the cathode).

$\begin{matrix}{{\sum\limits_{k = 2}^{j - 1}\;{n_{k}d_{k}}} \leq \frac{\left( {{2m} - 1} \right)\lambda}{4} \leq {{n_{1}d_{1}} + {\sum\limits_{k = 2}^{j - 1}\;{n_{k}d_{k}}}}} & \left( {{Formula}\mspace{14mu} 1} \right)\end{matrix}$

Here, it is necessary to adjust the film thickness in order to satisfythe formula (1). Since the layer mainly containing the organic compoundhas low electron mobility, the drive voltage increases when theelectron-transporting material and the electron-generating layer 105 inwhich electrons serve as the carrier are thick. Consequently, in thisembodiment mode, the thickness of the second hole-generating layer 103in which the mobility is comparatively high in the layer mainlycontaining the organic compound is adjusted, whereby the formula (1) issatisfied without drastically increasing the drive voltage.

In the case of a light-emitting element in which light is extracted fromthe side of the cathode 106, the light is reflected at the anode 101.Therefore, in order to increase the current efficiency of thelight-emitting element and to decrease the change of the spectrum shapewhich occurs depending on the angle of viewing the surface from whichthe light is extracted, the optical distance (refractive index×physicaldistance) from the light-emission position to the surface of the anode101 may be set to (2m−1)λ/4 (m is a natural number of 1 or more and λ isa center wavelength of the light emitted from the light-emitting layer).

The light-emitting layer 104 may be formed either in a single-layerstructure of the layer containing the light-emitting material or in amultilayer structure including the layer containing the light-emittingmaterial and the layer such as the electron-transporting layer or thehole-transporting layer. The layer containing the light-emittingmaterial may be a layer in which the light-emitting material to be thelight-emission center is diffused, or a layer consisting of thelight-emitting material. However, in any one of the above-mentionedstructures, the layer containing the light-emitting material has acertain degree of thickness and an infinite number of the luminescencecenters exist; therefore, it is impossible to determine the exactposition where the light emission occurs. Accordingly, in thisembodiment mode, a position of a part of the film containing thelight-emitting material that corresponds to a half thickness thereof isregarded as the position where the light-emission occurs.

One or a plurality of layers are provided between the position where thelight-emission occurs and the anode 101. In this embodiment mode, thelayer corresponds to the first hole-generating layer 102. Further, itcan be said that a part of the layer containing the light-emittingmaterial that corresponds to a half thickness thereof is also the layerlocated between the position where the light-emission occurs and theanode 101. Moreover, more layers may be included in the case where thelight-emitting layer is formed with a plurality of layers. In such astructure, the optical distance from the light-emission position to theanode 101 can be calculated by multiplying the thicknesses and therefractive indexes of the respective films and summing up the products.That is to say, the following formula (2) is satisfied. In the formula(2), the layer containing the light-emitting material is assumed to be 1and the anode 101 is assumed to be j (j is an integer number of 4 ormore), and the layers existing between the layer containing thelight-emitting material and the anode 101 are denoted with numerals inorder from the layer containing the light-emitting material. Moreover,the refractive index n and the thickness d with a certain numeral giventhereto indicate the refractive index and the thickness of the layer towhich the same numeral is given (that is, n₁ is the refractive index ofthe layer containing the light-emitting material and d_(j) is thethickness of the anode).

$\begin{matrix}{{\sum\limits_{k = 2}^{j - 1}\;{n_{k}d_{k}}} \leq \frac{\left( {{2m} - 1} \right)\lambda}{4} \leq {{n_{1}d_{1}} + {\sum\limits_{k = 2}^{j - 1}\;{n_{k}d_{k}}}}} & \left( {{Formula}\mspace{14mu} 2} \right)\end{matrix}$

Here, it is necessary to adjust the film thickness in order to satisfythe formula (2). In this embodiment mode, the formula (2) can besatisfied without drastically increasing the drive voltage, by adjustingthe thickness of the first hole-generating layer 102 in which themobility is comparatively high in the layer mainly containing theorganic compound.

In the case of the structure in which light is extracted from both ofthe anode 101 and the cathode 106, both of the formulas (1) and (2) maybe satisfied.

With the structure of the light-emitting element shown in thisembodiment mode, it is possible to provide a light-emitting element inwhich the change of the light-emission spectrum which occurs dependingon the angle of viewing the surface from which the light is extracted isdecreased.

The present embodiment mode can be combined with Embodiment Mode 1.

Embodiment Mode 3

This embodiment mode describes a display device according to the presentinvention shown in Embodiment Mode 1 or Embodiment Mode 2 while showingits manufacturing method with reference to FIGS. 5A to 6C. Although thisembodiment mode shows an example of manufacturing an active matrixdisplay device, a light-emitting element of the present invention isalso applicable for a passive matrix display device.

First, a first base insulating layer 51 a and a second base insulatinglayer 51 b are formed over a substrate 50, and then a semiconductorlayer is formed over the second base insulating layer 51 b (FIG. 5A).

As a material of the substrate 50, glass, quartz, plastic (such aspolyimide, acrylic, polyethylene terephthalate, polycarbonate,polyacrylate, or polyethersulfone), or the like can be used. Thesesubstrates may be used after being polished by CMP or the like asnecessary. In this embodiment mode, a glass substrate is used.

The first base insulating layer 51 a and the second base insulatinglayer 51 b are provided in order to prevent an element which adverselyaffects the characteristic of the semiconductor film such as alkalimetal or alkali-earth metal in the substrate 50 from diffusing into thesemiconductor layer. As the material of these base insulating layers,silicon oxide, silicon nitride, silicon oxide containing nitrogen,silicon nitride containing oxygen, or the like can be used. In thisembodiment mode, the first base insulating layer 51 a is formed withsilicon nitride, and the second base insulating layer 51 b is formedwith silicon oxide. Although the base insulating layer is formed in atwo-layer structure including the first base insulating layer 51 a andthe second base insulating layer 51 b in this embodiment mode, the baseinsulating layer may be formed in a single-layer structure or amultilayer structure including three or more layers. The base insulatinglayer is not necessary when the diffusion of the impurity from thesubstrate does not lead to a significant problem.

In this embodiment mode, the semiconductor layer formed subsequently isobtained by crystallizing an amorphous silicon film with a laser beam.The amorphous silicon film is formed in 25 to 100 nm thick (preferably30 to 60 nm thick) over the second base insulating layer 51 b by a knownmethod such as a sputtering method, a reduced-pressure CVD method, or aplasma CVD method. After that, heat treatment is conducted for one hourat 500° C. for dehydrogenation.

Next, the amorphous silicon film is crystallized with a laserirradiation apparatus to form a crystalline silicon film. In thisembodiment mode, an excimer laser is used at the laser crystallization.After the emitted laser beam is shaped into a linear beam spot using anoptical system, the amorphous silicon film is irradiated with the linearbeam spot. Thus, the crystalline silicon film is formed which is to beused as the semiconductor layer.

Alternatively, the amorphous silicon film can be crystallized by anothermethod such as a method in which the crystallization is conducted onlyby heat treatment or a method in which heat treatment is conducted usinga catalyst element for inducing the crystallization. As the element forinducing the crystallization, nickel, iron, palladium, tin, lead,cobalt, platinum, copper, gold, or the like is given. By using such anelement, the crystallization is conducted at lower temperature inshorter time than the crystallization only by the heat treatment;therefore, the damage to the glass substrate is suppressed. In the caseof crystallizing only by the heat treatment, a quartz substrate whichcan resist the high temperature is preferably used as the substrate 50.

Subsequently, a small amount of impurity elements are added to thesemiconductor layer as necessary in order to control the threshold,which is so-called channel doping. In order to obtain the requiredthreshold, an impurity showing N-type or P-type (such as phosphorus orboron) is added by an ion-doping method or the like.

After that, as shown in FIG. 5A, the semiconductor layer is patternedinto a predetermined shape so that an island-shaped semiconductor layer52 is obtained. The patterning is conducted by etching the semiconductorlayer using a mask. The mask is formed in such a way that a photo resistis applied to the semiconductor layer and the photo resist is exposedand baked so that a resist mask having a desired mask pattern is formedover the semiconductor layer.

Next, a gate insulating layer 53 is formed so as to cover thesemiconductor layer 52. The gate insulating layer 53 is formed in 40 to150 nm thick with an insulating layer containing silicon by a plasma CVDmethod or a sputtering method. In this embodiment mode, silicon oxide isused.

Then, a gate electrode 54 is formed over the gate insulating layer 53.The gate electrode 54 may be formed with an element selected from thegroup consisting of tantalum, tungsten, titanium, molybdenum, aluminum,copper, chromium, and niobium, or may be formed with an alloy materialor a compound material which contains the above element as its maincomponent. Further, a semiconductor film typified by a poly-crystallinesilicon film doped with an impurity element such as phosphorus may beused. Ag—Pd—Cu alloy may also be used.

Although the gate electrode 54 is formed with a single layer in thisembodiment mode, the gate electrode 54 may have a multilayer structureincluding two or more layers of, for example, tungsten as a lower layerand molybdenum as an upper layer. Even in the case of forming the gateelectrode in the multilayer structure, the above-mentioned material ispreferably used. The combination of the above materials may also beselected appropriately. The gate electrode 54 is processed by etchingwith the use of a mask formed with a photo resist.

Subsequently, impurities are added to the semiconductor layer 52 at highconcentration using the gate electrode 54 as the mask. According to thisstep, a thin film transistor 70 comprising the semiconductor layer 52,the gate insulating layer 53, and the gate electrode 54 is formed.

The manufacturing process of the thin film transistor is not limited inparticular, and may be modified appropriately so that a transistorhaving a desired structure can be manufactured.

Although this embodiment mode employs a top-gate thin film transistorusing the crystalline silicon film obtained by the lasercrystallization, a bottom-gate thin film transistor using an amorphoussemiconductor film can also be applied to a pixel portion. Not onlysilicon but also silicon germanium can be used for the amorphoussemiconductor. In the case of using silicon germanium, the concentrationof germanium preferably ranges from approximately 0.01 to 4.5 atomic %.

Moreover, a microcrystal semiconductor (semi-amorphous semiconductor)film which includes crystal grains each having a diameter of 0.5 to 20nm in the amorphous semiconductor may also be used. The microcrystalhaving the crystal with a diameter of 0.5 to 20 nm is also referred toas a so-called microcrystal (μc).

Semi-amorphous silicon (also referred to as SAS), which belongs to thesemi-amorphous semiconductor, can be obtained by decomposing silicidegas according to glow discharging. As typical silicide gas, SiH₄ isgiven. Besides, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like can beused. By using the silicide gas after diluting the silicide gas withhydrogen or hydrogen and one or plural kinds of noble gas selected fromthe group consisting of helium, argon, krypton, and neon, SAS can beeasily formed. The silicide gas is preferably diluted with the dilutionratio of 1:10 to 1:1000. The reaction to form the film by thedecomposition according to glow discharging may be conducted at thepressure ranging from 0.1 to 133 Pa. The electric power for forming theglow discharging may be supplied at high frequency in the range of 1 to120 MHz, preferably 13 to 60 MHz. The substrate heat temperature ispreferably 300° C. or less, preferably in the range of 100 to 250° C.

The raman spectrum of thus formed SAS shifts to the side of lowerwavenumber than 520 cm⁻¹. According to X-ray diffraction, diffractionpeaks of a silicon crystal lattice are observed at (111) and (220). As aterminating agent of a dangling bond, hydrogen or halogen is added by atleast 1 atomic % or more. As the impurity element in the film, theimpurity in the air such as oxygen, nitrogen, and carbon is desirably1×10²⁰ cm⁻¹ or less, and especially, the concentration of oxygen is5×10¹⁹/cm³ or less, preferably 1×10¹⁹/cm³ or less. The mobility of a TFTmanufactured with this film is μ=1 to 10 cm²/Vsec.

This SAS may be used after being crystallized further with a laser beam.

Subsequently, an insulating film (hydride film) 59 is formed withsilicon nitride so as to cover the gate electrode 54 and the gateinsulating layer 53. After forming the insulating film (hydride film)59, heat treatment for approximately 1 hour at 480° C. is conducted soas to activate the impurity element and to hydrogenate the semiconductorlayer 52.

Subsequently, a first interlayer insulating layer 60 is formed so as tocover the insulating film (hydride film) 59. As a material for formingthe first interlayer insulating layer 60, silicon oxide, acrylic,polyimide, siloxane, a low-k material, or the like is preferably used.In this embodiment mode, a silicon oxide film is formed as the firstinterlayer insulating layer. In this specification, siloxane is amaterial whose skeletal structure includes a bond of silicon and oxygenand which has an organic group containing at least hydrogen (such as analkyl group or an aryl group), a fluoro group, or the organic groupcontaining at least hydrogen and the fluoro group as the substituent(FIG. 5B).

Next, contact holes that reach the semiconductor layer 52 are formed.The contact holes can be formed by etching with a resist mask until thesemiconductor layer 52 is exposed. Either wet etching or dry etching canbe applied. The etching may be conducted once or multiple timesdepending on the condition. When the etching is conducted multipletimes, both of the wet etching and the dry etching may be conducted(FIG. 5C).

Then, a conductive layer is formed so as to cover the contact holes andthe first interlayer insulating layer 60. A connection portion 61 a, awiring 61 b, and the like are formed by processing the conductive layerinto a desired shape. This wiring may be a single layer of aluminum,copper, or the like. In this embodiment mode, the wiring is formed in amultilayer structure of molybdenum/aluminum/molybdenum in order from thebottom. Alternatively, a structure of titanium/aluminum/titanium ortitanium/titanium nitride/aluminum/titanium is also applicable (FIG.5D).

A second interlayer insulating layer 63 is formed so as to cover theconnection portion 61 a, the wiring 61 b, and the first interlayerinsulating layer 60. As the material of the second interlayer insulatinglayer 63, an applied film having self-flattening properties such as afilm of acrylic, polyimide, siloxane, or the like is preferable. In thisembodiment mode, the second interlayer insulating layer 63 is formedwith siloxane (FIG. 5E).

Next, an insulating layer may be formed with silicon nitride over thesecond interlayer insulating layer 63. This is to prevent the secondinterlayer insulating layer 63 from being etched more than necessary ina later step of etching a pixel electrode. Therefore, the insulatinglayer is not necessary in particular when the difference of the etchingrate is large between the pixel electrode and the second interlayerinsulating layer. Next, a contact hole penetrating the second interlayerinsulating layer 63 to reach the connection portion 61 a is formed.

Next, after a light-transmitting conductive layer is formed so as tocover the contact hole and the second interlayer insulating layer 63 (orthe insulating layer), the light-transmitting conductive layer isprocessed to form the anode 101 of the thin film light-emitting element.Here, the anode 101 electrically contacts the connection portion 61 a.As the material of the anode 101, it is preferable to use metal, alloy,an electrically conductive compound, or mixture of these each of whichhas high work function (work function of 4.0 eV or more). For example,ITO (indium tin oxide), ITO containing silicon (ITSO), IZO (indium zincoxide) in which zinc oxide (ZnO) is mixed by 2 to 20% into indium oxide,zinc oxide, GZO (gallium zinc oxide) in which gallium is contained inzinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W),chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu),palladium (Pd), or metal nitride such as TiN can be used. In thisembodiment mode, the anode 101 is formed with ITSO (FIG. 6A).

Next, an insulating layer formed with an organic material or aninorganic material is formed so as to cover the second interlayerinsulating layer 63 (or the insulating layer) and the anode 101.Subsequently, the insulating layer is processed so as to partiallyexpose the anode 101, thereby forming a partition wall 65. As thematerial of the partition wall 65, a photosensitive organic material(such as acrylic or polyimide) is preferable. Besides, anon-photosensitive organic material or inorganic material may also beused. Further, the partition wall 65 may be used as a black matrix bymaking the partition wall 65 black in such a way that a black pigment ordye such as titanium black or carbon nitride is diffused into thematerial of the partition wall 65 with the use of a diffuse material. Itis desirable that the partition wall 65 has a tapered shape in its endsurface toward the first electrode with its curvature changingcontinuously (FIG. 6B).

Next, a light-emitting laminated body 66 is formed so as to cover a partof the anode 101 that is exposed from the partition wall 65. In thisembodiment mode, the light-emitting laminated body 66 may be formed byan evaporating method or the like. The light-emitting laminated body 66is formed with the first hole-generating layer 102, the light-emittinglayer 104, the electron-generating layer 105, and the secondhole-generating layer 103 stacked in order.

The first hole-generating layer 102 and the second hole-generating layer103 may be formed with either different materials or the same material.For example, a layer containing both of a hole-transporting material andan electron-accepting material which can receive electrons from thehole-transporting material, a P-type semiconductor layer, or a layercontaining a P-type semiconductor is used. As the hole-transportingmaterial, for example, an aromatic amine compound (having a bond of abenzene ring with nitrogen), phthalocyanine (abbreviated to H2Pc), or aphthalocyanine compound such as copper phthalocyanine (abbreviated toCuPc) or vanadyl phthalocyanine (abbreviated to VOPc) can be used. Thearomatic amine compound is, for example,4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated to α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (abbreviated toTPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviated toTDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]triphenylamine(abbreviated to MTDATA), or4,4′-bis(N-(4-(N,N-di-m-tolylamino)phenyl)-N-phenylamino)biphenyl(abbreviated to DNTPD). As the electron-accepting material which canreceive electrons from these hole-transporting materials, for example,the following can be used; molybdenum oxide, vanadium oxide,7,7,8,8,-tetracyanoquinodimethane (abbreviated to TCNQ),2,3-dicyanonaphtoquinone (abbreviated to DCNNQ),2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (abbreviated toF4-TCNQ), or the like. The electron-accepting material is selected whichcan receive electrons in accordance with the combination with thehole-transporting material. Further, molybdenum oxide, vanadium oxide,ruthenium oxide, cobalt oxide, nickel oxide, or copper oxide can be usedas the P-type semiconductor. It is to be noted that the materialsmentioned above are just examples, and a practitioner can selectappropriately. Concerning the hole-transporting material and theelectron-accepting material which can receive electrons from thehole-transporting material, the mixture ratio of the electron-acceptingmaterial to the hole-transporting material is preferably 0.5 or more,more preferably in the range of 0.5 to 2, in molar ratio. In thisembodiment mode, the first hole-generating layer and the secondhole-generating layer use α-NPD as the electron-transporting materialand use molybdenum oxide (MoO₃) as the electron-accepting material whichcan receive electrons from α-NPD. α-NPD and MoO₃ are deposited by aco-evaporating method so that the mass ratio is α-NPD:MoO₃=4:1 (whichcorresponds to 1 in molar ratio). In this embodiment mode, the firsthole-generating layer is formed in 50 nm thick and the secondhole-generating layer is formed in 20 nm thick.

When the light-emitting layer 104 is formed with a layer in which alight-emitting material to be the light-emission center is diffused inthe layer containing the material having larger energy gap than thelight-emitting material, the following material can be used as thelight-emitting material to be the luminescence center;4-dicyanomethylene-2-methyl-6[-2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl)-4H-pyran(abbreviation: DCJT);4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]-4H-pyran;periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyl-julolidine-9-yl)ethenyl]benzene,N,N′-dimethylquinacridone (abbreviated to DMQd), coumarin 6, coumarin545T, tris(8-quinolinolato)aluminum (abbreviated to Alq₃),9,9′-bianthryl, 9,10-diphenylanthracene (abbreviated to DPA),9,10-bis(2-naphthyl)anthracene (abbreviated to DNA),2,5,8,11-tetra-t-butylperylene (abbreviated to TBP), or the like. As thematerial to be a base material in which the light-emitting material isdiffused, the following can be used; an anthracene derivative such as9,10-di(2-naphtyl)-2-tert-butylanthracene (abbreviated to t-BuDNA), acarbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl (abbreviatedto CBP), or a metal complex such as tris(8-quinolinolato)aluminum(abbreviated to Alq₃), tris(4-methyl-8-quinolinolato)aluminum(abbreviated to Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviated to BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq), bis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviated to Znpp₂), orbis[2-(2-hydroxyphenyl)benzoxazolate]zinc (abbreviated to ZnBOX). As thematerial which can constitute the light-emitting layer 104 singularly,tris(8-quinolinolato)aluminum (abbreviated to Alq₃),9,10-bis(2-naphtyl)anthracene (abbreviated to DNA),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviated toBAlq), or the like can be used.

The light-emitting layer 104 may be formed either in a single-layerstructure or a multilayer structure. Moreover, a hole-transporting layermay be provided between the first hole-generating layer 102 and thelayer in which the light-emitting material is diffused in thelight-emitting layer 104 (or the layer containing the light-emittingmaterial). Further, an electron-transporting layer may be providedbetween the electron-generating layer 105 and the layer in which thelight-emitting material is diffused in the light-emitting layer 104 (orthe layer containing the light-emitting material). These layers are notalways necessary to be provided, and only one or both of thehole-transporting layer and the electron-transporting layer may beprovided. The materials of the hole-transporting layer and theelectron-transporting layer conform to those of the hole-transportinglayer in the hole-generating layer and the electron-transporting layerin the electron-generating layer; therefore, the description is omittedhere. Refer to the description of those layers.

In this embodiment mode, the hole-transporting layer, the layer in whichthe light-emitting material is diffused, and the electron-transportinglayer are formed as the light-emitting layer 104 over thehole-generating layer 102. α-NPD is deposited in 10 nm thick as thehole-transporting layer, Alq and coumarin 6 are deposited in 35 nm thickwith their mass ratio of 1:0.005 as the layer in which thelight-emitting material is diffused, and Alq is deposited in 10 nm thickas the electron-transporting layer.

As the electron-generating layer 105, a layer containing both of anelectron-transporting material and an electron-donating material whichcan donate electrons to the electron-transporting material, an N-typesemiconductor layer, or a layer containing an N-type semiconductor canbe used. As the electron-transporting material, for example, thefollowing can be employed; a material containing a metal complex whichhas a quinoline skeleton or a benzoquinoline skeleton such astris-(8-quinolinolato)aluminum (abbreviated to Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviated to Almq₃),bis(10-hydroxybenzo[h]-quinolinolato)beryllium (abbreviated to BeBq₂),or bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviatedto BAlq). Besides, metal containing a metal complex which has an oxazoleor thiazole ligand such as bis[2-(2-hydroxyphenyl)benzoxazolate]zinc(abbreviated to Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolate]zinc(abbreviated to Zn(BTZ)₂) can be used. In addition to the metal complex,the following can be employed;2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviated toPBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated to OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated to p-EtTAZ), bathophenanthroline (abbreviated to BPhen),bathocuproin (abbreviated to BCP), or the like. As the electron-donatingmaterial which can donate electrons to the electron-transportingmaterial, for example, alkali metal such as lithium or cesium,magnesium, alkali-earth metal such as calcium, or rare-earth metal suchas erbium or ytterbium can be used. The electron-donating material whichcan donate electrons is selected in accordance with the combination withthe electron-transporting material. Further, zinc oxide, zinc sulfide,zinc selenide, titanium oxide, or the like can be used as the N-typesemiconductor.

The mixture ratio between the electron-transporting material and theelectron-donating material which can donate electrons to theelectron-transporting material is approximately 1:0.5 to 1:2, preferably1:1, in molar ratio. The electron-transporting material is formed withAlq and the electron-donating material which can donate electrons to Alqis formed with lithium (Li) in the electron-generating layer in thisembodiment mode. Deposition is conducted by the co-evaporating method sothat Alq:Li=1:0.01 in mass ratio. The film thickness is set to 10 nm.

The light-emitting element which emits light with a different emissionwavelength may be formed for each pixel to conduct the color display.Typically, light-emitting elements corresponding to the respectivecolors of R (red), G (green), and B (blue) are formed. Even in thiscase, the color purity can be increased and the pixel portion can beprevented from having a mirror surface (reflection) by providing, at aside of the pixel from which the light is emitted, a filter (coloredlayer) which transmits light of the emission wavelength. By providingthe filter (colored layer), a circular polarizing plate which has beenrequired conventionally can be omitted and the loss of the light emittedfrom the light-emitting element can be suppressed. Moreover, the changeof the tone of color when viewing the pixel portion (display screen)obliquely can be reduced.

The light-emitting element can have a structure in which light of asingle color or a white color is emitted. In the case of using thelight-emitting element of the white color, a filter (colored layer) totransmit the light of a particular wavelength is provided at a side ofthe pixel from which the light is emitted. Thus, the color display canbe conducted.

In order to form a light-emitting layer which emits the white light, thewhite light can be obtained by stacking, for example, Alq₃; Alq₃partially doped with nile red, which is pigment for red light emission,Alq₃, p-EtTAZ, and TPD (aromatic diamine) sequentially according to anevaporating method.

Moreover, the light-emitting layer may be formed not only with asinglet-excited light-emitting material but also with a triplet-excitedlight-emitting material containing a metal complex and the like. Forexample, among a red light-emitting pixel, a green light-emitting pixel,and a blue light-emitting pixel, the red light-emitting pixel, which hasa comparatively short half-life period, is formed with the tripletexcited light-emitting material, and the others are formed with thesinglet-excited light-emitting material. Because of the high emissionefficiency, the triplet-excited light-emitting material has thecharacteristic that the power consumption is low to obtain the sameluminance. That is to say, in the case of applying the triplet-excitedlight-emitting material to the red pixel, the amount of current flowingto the light-emitting element is small; therefore, the reliability canbe enhanced. For lower power consumption, the red light-emitting pixeland the green light-emitting pixel may be formed with thetriplet-excited light-emitting material, and the blue light-emittingpixel may be formed with the singlet-excited light-emitting material. Byforming the green light-emitting element, to which human visibility ishigh, also with the triplet-excited light-emitting material, lower powerconsumption can be achieved.

As an example of the triplet-excited light-emitting material, thefollowing can be employed; a material using a metal complex as dopant,such as a metal complex containing platinum, which is one of thirdtransition elements, as metal center or a metal complex containingiridium as metal center. The triplet-excited light-emitting material isnot limited to these compounds, and other compounds which have the abovestructure and contain an element belonging to any one of groups 8 to 10in the periodic table as metal center can be used.

The light-emitting element formed with the above material emits light byapplying bias in a forward direction. A pixel of a display device formedusing the light-emitting element can be driven by a simple matrix methodor an active matrix method. In any way, the respective pixels emit lightby applying bias in the forward direction at a particular timing and donot emit light for a certain period. The reliability of thelight-emitting element can be increased by applying bias in a reversedirection in this non-emission period. The light-emitting element hasdeterioration modes in which light-emission intensity decreases under acertain drive condition or the luminance seems to decrease because anon-emission region expands within the pixel. However, when alternatelydriving is conducted by applying bias in the forward and reversedirections, the progress of the deterioration can be slowed down and thereliability of the light-emitting device can be enhanced.

Subsequently, the cathode 106 is formed so as to cover thelight-emitting laminated body 66. Accordingly, a light-emitting element93 comprising the anode 101, the light-emitting laminated body 66, andthe cathode 106 can be manufactured. As the cathode material used forforming the cathode 106, it is preferable to use metal, alloy, anelectrically conductive compound, mixture of these, or the like each ofwhich has low work function (work function of 3.8 eV or less). As aspecific example of the cathode material, the following can be given; anelement belonging to group 1 or 2 in the periodic table, that is, alkalimetal such as Li or Cs, Mg, alkali-earth metal such as Ca or Sr, alloycontaining these elements such as Mg:Ag or Al:Li, or a compoundcontaining these elements such as LiF, CsF, or CaF₂. In addition, thecathode can also be formed with a transition metal containing rare-earthmetal. Further, a multilayer containing the above element and anothermetal (including alloy) such as Al, Ag, or ITO can be used. In thisembodiment mode, the cathode is formed with aluminum.

In the light-emitting element having the above structure, the drivevoltage is low and the increase in the drive voltage over time is small.

The electrode in electrical contact with the connection portion 61 a isthe anode 101 in this embodiment mode; however, the electrode inelectrical contact with the connection portion 61 a may be the cathode106. In this case, the light-emitting laminated body 66 may be formed bystacking the second hole-generating layer 103, the electron-generatinglayer 105, the light-emitting layer 104, and the first hole-generatinglayer 102 in order, and the anode 101 may be formed over thelight-emitting laminated body 66.

After that, a silicon oxynitride film is formed as a second passivationfilm by a plasma CVD method. In the case of using the silicon oxynitridefilm, a silicon oxynitride film manufactured with SiH₄, N₂O, and NH₃ bya plasma CVD method, a silicon oxynitride film manufactured with SiH₄and N₂O by a plasma CVD method, or a silicon oxynitride filmmanufactured with gas in which SiH₄ and N₂O are diluted with Ar by aplasma CVD method is preferably formed.

As a first passivation film, a silicon oxynitride hydride filmmanufactured with SiH₄, N₂O, and H₂ is also applicable. The structure ofthe first passivation film is not limited to the single-layer structure,and the first passivation film may be formed in a single-layer structureor a multilayer structure of another insulating layer containingsilicon. A multilayer film of a carbon nitride film and a siliconnitride film, a multilayer film of styrene polymer, a silicon nitridefilm, or a diamond-like carbon film may be formed instead of the siliconoxynitride film.

Subsequently, in order to protect the light emitting element from thedeterioration-promoting material such as moisture, the display portionis sealed. In the case of using a counter substrate for sealing, thecounter substrate and an element substrate are pasted together by aninsulating sealing material so as to expose an external connectionportion. The space between the counter substrate and the elementsubstrate may be filled with inert gas such as dry nitrogen, or thesealing material may be applied to the whole surface of the pixelportion for pasting the counter substrate. It is preferable to use anultraviolet curable resin or the like as the sealing material. A dryingagent or particles for keeping the gap between the substrates uniformmay be mixed into the sealing material. Subsequently, a flexible wiringsubstrate is pasted to the external connection portion, therebycompleting the display device.

An example of the structure of the thus manufactured display device isdescribed with reference to FIGS. 7A and 7B. Although the shapes aredifferent, the same parts having the same function are denoted with thesame reference numerals and the description to such parts may beomitted. In this embodiment mode, the thin film transistor 70 having theLDD structure connects to the light-emitting element 93 via theconnection portion 61 a.

In FIG. 7A, the anode 101 is formed with a light-transmitting conductivefilm and has a structure in which light emitted from the light-emittinglaminated body 66 is extracted to the side of the substrate 50. Areference numeral 94 denotes a counter substrate, which is to be fixedto the substrate 50 with a sealing material or the like after formingthe light-emitting element 93. By filling the space between the countersubstrate 94 and the element with a light-transmitting resin 88 or thelike and sealing the space, it is possible to prevent the light-emittingelement 93 from deteriorating due to the moisture. Further, the resin 88desirably has moisture-absorption properties. In addition, it is moredesirable that a drying agent 89 having high light-transmittingproperties is diffused in the resin 88 because the effect of moisturecan be suppressed further.

In FIG. 7B, the anode 101 and the cathode 106 are both formed with alight-transmitting conductive film and have a structure in which lightcan be extracted toward both of the substrate 50 and the countersubstrate 94. In this structure, it is possible to prevent the screenfrom becoming transparent by providing a polarizing plate 90 outside thesubstrate 50 and the counter substrate 94, whereby increasing thevisibility. A protective film 91 is preferably provided outside thepolarizing plate 90.

Either an analog video signal or a digital video signal may be used inthe display device having the display function according to the presentinvention. The digital video signal includes a video signal usingvoltage and a video signal using current. When the light-emittingelement emits light, the video signal inputted into the pixel uses theconstant voltage or the constant current. When the video signal uses theconstant voltage, the voltage applied to the light-emitting element orthe current flowing in the light-emitting element is constant. On theother hand, when the video signal uses the constant current, the voltageapplied to the light-emitting element or the current flowing in thelight-emitting element is constant. The light-emitting element to whichthe constant voltage is applied is driven by the constant voltage, andthe light-emitting element in which the constant current flows is drivenby the constant current. The constant current flows in thelight-emitting element driven by the constant current without beingaffected by the change of the resistance of the light-emitting element.Either method may be employed in the light-emitting display device andits driving method of the present invention.

In the display device according to the present invention manufactured bythe method in this embodiment mode, the drive voltage is low and theincrease in the drive voltage over time is small.

Embodiment Mode 4

This embodiment mode describes an external view of a panel of alight-emitting device corresponding to one aspect of the presentinvention with reference to FIGS. 8A and 8B. FIG. 8A is a top view of apanel in which a transistor and a light-emitting element formed over asubstrate are sealed by a sealing material formed between the substrateand a counter substrate 4006. FIG. 8B corresponds to a cross-sectionalview of FIG. 8A. The structure of the light-emitting element mounted inthis panel is a structure in which a layer in contact with an electrodeis a hole-generating layer and a light-emitting layer is sandwichedbetween the hole-generating layers. Moreover, in the light-emittingelement, an electron-generating layer is provided between thehole-generating layer on a cathode side and the light-emitting layer.

A sealing material 4005 is provided so as to surround a pixel portion4002, a signal line driver circuit 4003, and a scanning line drivercircuit 4004 which are provided over a substrate 4001. In addition, thecounter substrate 4006 is provided over the pixel portion 4002, thesignal line driver circuit 4003, and the scanning line driver circuit4004. Therefore, the pixel portion 4002, the signal line driver circuit4003, and the scanning line driver circuit 4004 are sealed together witha filling material 4007 by the substrate 4001, the sealing material4005, and the counter substrate 4006.

The pixel portion 4002, the signal line driver circuit 4003, and thescanning line driver circuit 4004 provided over the substrate 4001 havea plurality of thin film transistors. FIG. 8B shows a thin filmtransistor 4008 included in the signal line driver circuit 4003 and athin film transistor 4010 included in the pixel portion 4002.

The light-emitting element 4011 is connected electrically to the thinfilm transistor 4010.

Further, a lead wiring 4014 corresponds to a wiring for supplying asignal or a power source voltage to the pixel portion 4002, the signalline driver circuit 4003, and the scanning line driver circuit 4004. Thelead wiring 4014 is connected to a connection terminal 4016 via a leadwiring 4015 a and a lead wiring 4015 b. The connection terminal 4016 iselectrically connected to a terminal of a flexible print circuit (FPC)4018 via an anisotropic conductive film 4019.

As the filling material 4007, in addition to inert gas such as nitrogenor argon, an ultraviolet curable resin or a thermoset resin can be used.For example, polyvinyl chloride, acrylic, polyimide, an epoxy resin, asilicon resin, polyvinyl butyral, or ethylene vinylene acetate can beused.

It is to be noted that the display device according to the presentinvention includes in its category the panel in which the pixel portionhaving the light-emitting element is formed and a module in which an ICis mounted in the panel.

In the panel and the module having the structure shown in thisembodiment mode, the drive voltage is low and the increase in the drivevoltage over time is small.

Embodiment Mode 5

As an electronic appliance according to the present invention to which amodule, for example the module which has been exemplified in EmbodimentMode 4, is mounted, the following is given; a camera such as a videocamera or a digital camera, a goggle type display (head mount display),a navigation system, a sound reproduction device (car audio component orthe like), a computer, a game machine, a mobile information terminal (amobile computer, a mobile telephone, a mobile game machine, anelectronic book, or the like), an image reproduction device equippedwith a recording medium (specifically a device which reproduces therecording medium such as a digital versatile disc (DVD) and which isequipped with a display for displaying the image), or the like. FIGS. 9Ato 9E show specific examples of these electronic appliances.

FIG. 9A shows a light-emitting display device, which corresponds to, forexample, a television receiving device or a monitor of a personalcomputer. The light-emitting display device according to the presentinvention includes a case 2001, a display portion 2003, speaker portions2004, and the like. In the light-emitting display device according tothe present invention, the drive voltage of the display portion 2003 islow and the increase in the drive voltage of the display portion 2003over time is small. In the pixel portion, a polarizing plate or acircular polarizing plate is preferably provided in the pixel portion toenhance the contrast. For example, films are preferably provided inorder of a quarter wave-plate, a half wave-plate, and a polarizing plateto a sealing substrate. Further, an anti-reflection film may be providedover the polarizing plate.

FIG. 9B shows a mobile phone including a main body 2101, a case 2102, adisplay portion 2103, an audio input portion 2104, an audio outputportion 2105, operation keys 2106, an antenna 2108, and the like. In thedisplay portion 2103 of the mobile phone according to the presentinvention, the drive voltage is low and the increase in the drivevoltage over time is small.

FIG. 9C shows a computer including a main body 2201, a case 2202, adisplay portion 2203, a keyboard 2204, an external connection port 2205,a pointing mouse 2206, and the like. In the display portion 2203 of thecomputer according to the present invention, the drive voltage is lowand the increase in the drive voltage over time is small. Although FIG.9C shows a laptop computer, the present invention is also applicable fora desktop computer in which a hard disk is integrated with a displayportion.

FIG. 9D shows a mobile computer including a main body 2301, a displayportion 2302, a switch 2303, operation keys 2304, an infrared port 2305,and the like. In the display portion 2302 of the mobile computeraccording to the present invention, the drive voltage is low and theincrease in the drive voltage over time is small.

FIG. 9E shows a mobile game machine including a case 2401, a displayportion 2402, speaker portions 2403, operation keys 2404, an recordingmedium insert portion 2405, and the like. In the display portion 2402 ofthe mobile game machine according to the present invention, the drivevoltage is low and the increase in the drive voltage over time is small.

As thus described, the present invention is applicable in a wide range,and can be used in electronic appliances of every field.

Embodiment Mode 6

FIGS. 10A to 10C show examples of bottom emission, dual emission, andtop emission, respectively. The structure whose manufacturing processhas been described in Embodiment Mode 2 corresponds to the structure ofFIG. 10C. FIGS. 10A and 10B show the structures in which a firstinterlayer insulating layer 900 in FIG. 10C is formed with a materialhaving self-flattening properties and a wiring to be connected with athin film transistor 901 and the anode 101 of the light-emitting elementare formed over the same interlayer insulating layer. In FIG. 10A, theanode 101 in the light-emitting element is formed with alight-transmitting material, and light is emitted toward a lower part ofthe light-emitting device, which is called a bottom-emission structure.In FIG. 10B, the cathode 106 is formed with a light-transmittingmaterial such as ITO, ITSO, or IZO, and light is extracted from bothsides, which is called a dual-emission structure. When a film is formedwith aluminum or silver thickly, the film does not transmit light;however, the film transmits light when the film is formed thinly.Therefore, by forming the cathode 106 with aluminum or silver in such athickness that light can pass therethrough, dual emission can beachieved.

Embodiment Mode 7

This embodiment mode describes a pixel circuit and a protective circuitin the panel and the module shown in Embodiment Mode 4, and theiroperations. FIGS. 5A to 6C show cross section of a driver TFT 1403 and alight-emitting element 1405 in FIGS. 11A to 11F.

A pixel shown in FIG. 11A includes a signal line 1410 and power sourcelines 1411 and 1412 in a column direction and a scanning line 1414 in arow direction. The pixel further includes a switching TFT 1401, thedriver TFT 1403, a current control TFT 1404, a capacitor element 1402,and the light-emitting element 1405.

A pixel shown in FIG. 11C has the same structure as that in FIG. 11Aexcept that a gate electrode of the driver TFT 1403 is connected to thepower source line 1412 provided in the row direction. In other words,the pixels shown in FIGS. 11A and 11C have the same equivalent circuitdiagram. However, in the case of arranging the power source line 1412 inthe column direction (FIG. 11A) and in the case of arranging the powersource line 1412 in the row direction (FIG. 11C), each power source lineis formed of a conductive film having a different layer. Here, attentionis paid to a wiring connected to the gate electrode of the driver TFT1403, and the structure is shown separately in FIGS. 11A and 11C inorder to show that the layers for manufacturing these wirings aredifferent.

As the characteristic of the pixels shown in FIGS. 11A and 11C, thedriver TFT 1403 and the current control TFT 1404 are connected seriallywithin the pixel, and it is preferable to set the channel length L(1403) and the channel width W (1403) of the driver TFT 1403, and thechannel length L (1404) and the channel width W (1404) of the currentcontrol TFT 1404 so as to satisfy L (1403)/W (1403):L (1404)/W (1404)=5to 6000:1.

The driver TFT 1403 operates in a saturation region and serves tocontrol the current value of the current flowing into the light-emittingelement 1405. The current control TFT 1404 operates in a linear regionand serves to control the current supply to the light-emitting element1405. Both TFTs preferably have the same conductivity type in themanufacturing step, and the TFTs are n-channel type TFTs in thisembodiment mode. The driver TFT 1403 may be either an enhancement typeor a depletion type. Since the current control TFT 1404 operates in thelinear region according to the present invention having the abovestructure, slight fluctuation of Vgs of the current control TFT 1404does not affect the current value of the light-emitting element 1405.That is to say, the current value of the light-emitting element 1405 canbe determined by the driver TFT 1403 operating in the saturation region.With the above structure, the unevenness of the luminance of thelight-emitting element due to the variation of the characteristic of theTFT can be improved, thereby providing a display device in which theimage quality is enhanced.

In the pixels shown in FIGS. 11A to 11D, the switching TFT 1401 is tocontrol the input of the video signal to the pixel, and the video signalis inputted into the pixel when the switching TFT 1401 is turned on.Then, the voltage of the video signal is held in the capacitor element1402. Although FIGS. 11A and 11C show the structure in which thecapacitor element 1402 is provided, the present invention is not limitedto this. When the gate capacitor and the like can cover the capacitorholding the video signal, the capacitor element 1402 is not necessarilyprovided.

A pixel shown in FIG. 11B has the same pixel structure as that in FIG.11A except that a TFT 1406 and a scanning line 1414 are added. In thesame way, a pixel shown in FIG. 11D has the same pixel structure as thatin FIG. 11C expect that the TFT 1406 and the scanning line 1414 areadded.

Switching of the TFT 1406 is controlled by the additionally providedscanning line 1414. When the TFT 1406 is turned on, the charge held inthe capacitor element 1402 is discharged, thereby turning off thecurrent control TFT 1404. In other words, by the provision of the TFT1406, a state can be produced compellingly in which the current is notflowed to the light-emitting element 1405. For this reason, the TFT 1406can be referred to as an eraser TFT. Consequently, in the structuresshown in FIGS. 11B and 11D, a lighting period can be started at the sametime as or just after the start of a writing period without waiting forthe writing of the signal into all the pixels; therefore the duty ratiocan be increased.

In a pixel shown in FIG. 11E, the signal line 1410 and the power sourceline 1411 are arranged in the column direction, and the scanning line1414 is arranged in the row direction. Further, the pixel includes theswitching TFT 1401, the driver TFT 1403, the capacitor element 1402, andthe light-emitting element 1405. A pixel shown in FIG. 11F has the samepixel structure as that shown in FIG. 7E except that the TFT 1406 and ascanning line 1415 are added. In the structure shown in FIG. 11F, theduty ratio can also be increased by the provision of the TFT 1406.

As thus described, various pixel circuits can be employed. Inparticular, in the case of forming a thin film transistor with anamorphous semiconductor film, the semiconductor film for the driver TFT1403 is preferably large. Therefore, in the above pixel circuit, a topemission type is preferable in which light from the light emitting layeris emitted from the side of the sealing substrate.

Such an active matrix light-emitting device can be driven at low voltagewhen the pixel density increases, because the TFTs are provided in eachpixel. Therefore, it is considered that the active matrix light-emittingdevice is advantageous.

Although this embodiment mode describes the active matrix light-emittingdevice in which the respective TFTs are provided in each pixel, apassive matrix light-emitting device can also be formed in which TFTsare provided for each column. Since the TFTs are not provided in eachpixel in the passive matrix light-emitting device, high aperture ratiocan be obtained. In the case of a light-emitting device in which lightis emitted to both sides of the electroluminescent layer, thetransmissivity of the passive matrix display device is increased.

In the display device further comprising such pixel circuits accordingto the present invention, the drive voltage is low and the increase inthe drive voltage over time is small. Moreover, the display device hasthe respective characteristics.

Subsequently, a case is described in which a diode is provided as aprotective circuit to the scanning line and the signal line with the useof an equivalent circuit shown in FIG. 11E.

In FIG. 12, the switching TFTs 1401 and 1403, the capacitor element1402, and the light-emitting element 1405 are provided in a pixelportion 1500. Diodes 1561 and 1562 are provided to the signal line 1410.In the similar way to the switching TFTs 1401 and 1403, the diodes 1561and 1562 are manufactured based on the above embodiment modes, and havea gate electrode, a semiconductor layer, a source electrode, a drainelectrode, and the like. The diodes 1561 and 1562 are operated as thediode by connecting the gate electrode with the drain electrode or thesource electrode.

Common potential lines 1554 and 1555 connecting with the diodes areformed with the same layer as the gate electrode. Therefore, in order toconnect with the source electrode or the drain electrode of the diode,it is necessary to form a contact hole in the gate insulating layer.

A diode provided to the scanning line 1414 has the similar structure.

As thus described, according to the present invention, a protectivediode to be provided to an input stage can be manufacturedsimultaneously. The position at which the protective diode is formed isnot limited to this, and the diode may also be provided between thedriver circuit and the pixel.

In the display device having such protective circuits according to thepresent invention, the increase in the drive voltage over time is smalland the reliability as the display device can be enhanced.

Embodiment 1

This embodiment shows measurement data of a light-emitting elementaccording to the present invention.

First, a manufacturing method of a light-emitting element in thisembodiment is described. The light-emitting element in this embodimentconforms to the structure of the light-emitting element shown inEmbodiment Mode 1. In this embodiment, a glass substrate is used as theinsulator 100. ITO containing silicon is formed over the glass substrateby a sputtering method, thereby forming the anode 101. The thickness ofthe anode 101 is set to 110 nm.

Subsequently, the first hole-generating layer 102 is formed withmolybdenum oxide and α-NPD by co-evaporating molybdenum oxide and α-NPDover the anode 101. Here, the thickness of the first hole-generatinglayer 102 is set to 50 nm.

Next, the light-emitting layer 104 is formed over the firsthole-generating layer 102. The light-emitting layer 104 is formed in athree-layer structure in which a hole-transporting layer, a layer wherea light-emitting material is diffused, and an electron-transportinglayer are stacked in order from the side of the first hole-generatinglayer 102. The hole-transporting layer is formed with α-NPD in 10 nmthick by a vacuum evaporating method. The layer in which thelight-emitting material is diffused is formed with Alq₃ and coumarin 6in 35 nm thick by a co-evaporating method. The electron-transportinglayer is formed with only Alq₃ in 10 nm thick by a vacuum evaporatingmethod. The layer in which the light-emitting material is diffused isadjusted so that the proportion between Alq₃ and coumarin 6 is 1:0.005in mass ratio.

Subsequently, the electron-generating layer 105 is formed with Alq₃ andlithium in 10 nm thick by co-evaporating Alq₃ and lithium over thelight-emitting layer 104. Alq₃ and lithium are adjusted so that the massratio between Alq₃ and lithium is 1:0.01.

Next, the second hole-generating layer 103 is formed with molybdenumoxide and α-NPD by co-evaporating molybdenum oxide and α-NPD over theelectron-generating layer 105. Here, the thickness of the firsthole-generating layer 102 is set to 20 nm. The molar ratio between α-NPDand molybdenum oxide is 1:1.

The cathode 106 is formed with aluminum in 100 nm thick over the secondhole-generating layer 105.

When voltage is applied to the light-emitting element having the abovestructure according to the present invention, holes are injected fromthe second hole-generating layer 103 to the second electrode. Moreover,electrons are injected from the electron-generating layer 105 to thelight-emitting layer 104. Further, holes are injected from the firsthole-generating layer 102 to the light-emitting layer 104. Then, theinjected holes and electrons are recombined in the light-emitting layer,thereby providing light from coumarin 6.

FIG. 13 shows the voltage-luminance characteristic of the thusmanufactured light-emitting element of this embodiment, while FIG. 14shows the voltage-current characteristic thereof. In FIG. 13, thehorizontal axis shows the voltage (V), and the vertical axis shows theluminance (cd/m²). In FIG. 14, the horizontal axis shows the voltage(V), and the vertical axis shows the current (mA).

Thus, the light-emitting element in this embodiment exhibits superiorcharacteristic.

FIG. 22A shows the absorption spectrum of a complex material containingα-NPD and molybdenum oxide used as the hole-generating layer in thisembodiment. FIG. 22B shows the absorption spectrum of only α-NPD andFIG. 22C shows the absorption spectrum of only molybdenum oxide. As isknown from the figures, the absorption spectrum of the complex materialcontaining α-NPD and molybdenum oxide has a peak which does not appearin the other absorption spectrums of only α-NPD and only molybdenumoxide. It is considered that this peak results from the generation ofholes by interaction of α-NPD and molybdenum oxide.

Embodiment 2

This embodiment describes a manufacturing method of four light-emittingelements having different mixture proportions between ahole-transporting material and an electron-accepting material whichshows electron-accepting properties to the hole-transporting material ina hole-generating layer. The four light-emitting elements are denoted bya light-emitting element (1), a light-emitting element (2), alight-emitting element (3), and a light-emitting element (4). Moreover,this embodiment describes the characteristics of these elements.

First, the manufacturing method of the light-emitting element in thisembodiment is described. In this embodiment, the light-emitting elementconforms to the structure of the light-emitting element shown inEmbodiment Mode 1. In this embodiment, a glass substrate is used as theinsulator 100. ITO containing silicon is formed over the glass substrateby a sputtering method, thereby forming the anode 101. The thickness ofthe anode 101 is set to 110 nm.

Subsequently, the first hole-generating layer 102 is formed withmolybdenum oxide over the anode 101 by a vacuum evaporating method.Here, the thickness of the first hole-generating layer 102 is set to 5nm.

Next, the light-emitting layer 104 is formed over the firsthole-generating layer 102. The light-emitting layer 104 is formed in athree-layer structure in which a hole-transporting layer, a layer wherea light-emitting material is diffused, and an electron-transportinglayer are stacked in order from the side of the first hole-generatinglayer 102. The hole-transporting layer is formed with α-NPD in 55 nmthick by a vacuum evaporating method. The layer in which thelight-emitting material is diffused is formed with Alq₃ and coumarin 6in 35 nm thick by a co-evaporating method. The electron-transportinglayer is formed with only Alq₃ in 10 nm thick by a vacuum evaporatingmethod. The layer in which the light-emitting material is diffused isadjusted so that the proportion between Alq₃ and coumarin 6 is 1:0.005in mass ratio.

Subsequently, the electron-generating layer 105 is formed with Alq₃ andlithium in 10 nm thick by co-evaporating Alq₃ and lithium over thelight-emitting layer 104. Alq₃ and lithium are adjusted so that the massratio between Alq₃ and lithium is 1:0.01.

Next, the second hole-generating layer 103 is formed with molybdenumoxide and α-NPD by co-evaporating molybdenum oxide and α-NPD over theelectron-generating layer 105. Here, the light-emitting element (1) isadjusted so that the molar ratio of α-NPD to molybdenum oxide is 0.5(=α-NPD/molybdenum oxide). The light-emitting element (2) is adjusted sothat the molar ratio of α-NPD to molybdenum oxide is 1.0(=α-NPD/molybdenum oxide). The light-emitting element (3) is adjusted sothat the molar ratio of α-NPD to molybdenum oxide is 1.5(=α-NPD/molybdenum oxide). The light-emitting element (4) is adjusted sothat the molar ratio of α-NPD to molybdenum oxide is 2.0(=α-NPD/molybdenum oxide). The thickness of the second hole-generatinglayer 102 is set to 20 nm.

The cathode 106 is formed with aluminum in 100 nm thick over the secondhole-generating layer 103.

When voltage is applied to the light-emitting element having the abovestructure according to the present invention, holes are injected fromthe second hole-generating layer 103 to the second electrode. Moreover,electrons are injected from the electron-generating layer 105 to thelight-emitting layer 104. Further, holes are injected from the firsthole-generating layer 102 to the light-emitting layer 104. Then, theinjected holes and electrons are recombined in the light-emitting layer,thereby providing light from coumarin 6.

FIG. 15 shows the voltage-luminance characteristic of the light-emittingelement in the present embodiment. FIG. 16 shows the currentdensity-luminance characteristic thereof, and FIG. 17 shows thevoltage-current characteristic thereof. In FIG. 15, the horizontal axisshows the voltage (V) and the vertical axis shows the luminance (cd/m²).In FIG. 16, the horizontal axis shows the current density (mA/cm²) andthe vertical axis shows the luminance (cd/m²). In FIG. 17, thehorizontal axis shows the voltage (V) and the vertical axis shows thecurrent (mA). In FIGS. 15 to 17, ▴ shows the characteristic of thelight-emitting element (1), ● shows the characteristic of thelight-emitting element (2),

shows the characteristic of the light-emitting element (3), and n showsthe characteristic of the light-emitting element (4).

It is to be understood from FIGS. 15 to 17 that all of thelight-emitting elements operate well. In the light-emitting elements (2)to (4) in which the molar ratio of α-NPD to molybdenum oxide(=α-NPD/molybdenum oxide) ranges from 1 to 2, high luminance is obtainedby applying any voltage and high current value is also obtained. Thus,the light-emitting element can be obtained which operates at lower drivevoltage by adjusting the molar ratio of α-NPD to molybdenum oxide(=α-NPD/molybdenum oxide) to be in the range of 1 to 2.

Next, a result of a continuously lighting test of the light-emittingelements of the present embodiment is described. After thelight-emitting element manufactured as above is sealed under nitrogenatmosphere, the continuously lighting test is conducted at normaltemperature in the following way.

As is clear from FIG. 16, the current density of 26.75 mA/cm² isrequired when the light is emitted with the luminance of 3000 cd/m² inan initial state of the light-emitting element of the present invention.In this embodiment, the current of 26.75 mA/cm² keeps to be flowed for acertain period of time, and data are collected on the change of thevoltage required to flow the current of 26.75 mA/cm² over time and thechange of the luminance over time. FIGS. 18 and 19 show the collecteddata. In FIG. 18, the horizontal axis shows the passed time (hour),while the vertical axis shows the voltage (V) required for flowing thecurrent of 26.75 mA/cm². In FIG. 19, the horizontal axis shows thepassed time (hour), while the vertical axis shows the luminance (anyunit of measure). It is to be noted that the luminance (any unit ofmeasure) is a relative value to the initial luminance expressed byassuming that the initial luminance be 100. The relative value isobtained in such a way that the luminance at a particular time isdivided by the initial luminance and multiplied by 100.

It is to be understood from FIG. 18 that after 100 hours have passed,the voltage required for flowing the current having the current densityof 26.75 mA/cm² is only approximately 1 V higher than that in theinitial state. This indicates that the light-emitting element of thepresent invention is a superior element in which the increase in thedrive voltage over time is small.

In the light-emitting elements shown in Embodiments 1 and 2, layersserving as the hole-injecting layer, the hole-transporting layer, theelectron-transporting layer, and the like are formed in addition to thelayer serving as the light-emitting layer. However, these layers are notalways necessary. Further, in Embodiments 1 and 2, after the layerserving as the light-emitting layer is formed, the electron-generatinglayer is formed, and then the hole-generating layer is formed. However,the manufacturing method of the light-emitting element according to thepresent invention is not limited to this. For example, after thehole-generating layer is formed, the electron-generating layer may beformed, and then the layer serving as the light-emitting layer may beformed.

Embodiment 3

This embodiment shows measurement data of a light-emitting elementaccording to the present invention which uses a different material fromEmbodiment 1.

First, a method for manufacturing a light-emitting element in thisembodiment is described. The light-emitting element in this embodimentconforms to the structure of the light-emitting element shown inEmbodiment Mode 1. In this embodiment, a glass substrate is used as theinsulator 100. ITO containing silicon is formed over the glass substrateby a sputtering method, thereby forming the anode 101 in 110 nm thick.

Subsequently, the first hole-generating layer 102 is formed withmolybdenum oxide and DNTPD by co-evaporating molybdenum oxide and DNTPDover the anode 101. Here, the thickness of the first hole-generatinglayer 102 is set to 50 nm. The mass ratio between DNTPD and molybdenumoxide is set to 2:1.

Next, the light-emitting layer 104 is formed over the firsthole-generating layer 102. The light-emitting layer 104 has athree-layer structure including a hole-transporting layer, a layer inwhich a light-emitting material is diffused, and anelectron-transporting layer in order from the side of the firsthole-generating layer 102. The hole-transporting layer is formed withα-NPD in 10 nm thick by a vacuum evaporating method. The layer in whichthe light-emitting material is diffused is formed with Alq₃ and coumarin6 in 35 nm thick by a co-evaporating method. The electron-transportinglayer is formed with only Alq₃ in 10 nm thick by a vacuum evaporatingmethod. It is to be noted that the layer in which the light-emittingmaterial is diffused is adjusted so that the proportion between Alq₃ andcoumarin 6 is 1:0.005 in mass ratio.

Subsequently, the electron-generating layer 105 is formed with Alq₃ andlithium in 10 nm thick by co-evaporating Alq₃ and lithium over thelight-emitting layer 104. The mass ratio between Alq₃ and lithium isadjusted so as to be 1:0.01.

Next, the second hole-generating layer 103 is formed with molybdenumoxide and DNTPD over the electron-generating layer 105 by co-evaporatingmolybdenum oxide and DNTPD. Here, the thickness of the firsthole-generating layer 102 is set to 20 nm. Further, the mass ratiobetween DNTPD and molybdenum oxide is adjusted so as to be 4:2.

The cathode 106 is formed with aluminum over the second hole-generatinglayer 103. The film thickness is set to 100 nm.

In the light-emitting element having the above structure according tothe present invention, holes are injected from the secondhole-generating layer 103 to the second electrode by applying voltage.Further, electrons are injected from the electron-generating layer 105to the light-emitting layer 104. Moreover, holes are injected from thefirst hole-generating layer 102 to the light-emitting layer 104. In thelight-emitting layer, the injected holes and electrons are recombined,thereby providing light from coumarin 6.

FIG. 20 shows the voltage-luminance characteristic of the thusmanufactured light-emitting element of this embodiment. FIG. 21 showsthe voltage-current characteristic thereof. In FIG. 20, the horizontalaxis shows the voltage (V), while the vertical axis shows the luminance(cd/m²). Meanwhile, in FIG. 21, the horizontal axis shows the voltage(V), while the vertical axis shows the current (mA).

Thus, the light-emitting element in this embodiment has superiorcharacteristic.

FIG. 23A shows the absorption spectrum of a complex material containingDNTPD and molybdenum oxide used as the hole-generating layer in thisembodiment. FIG. 23B shows the absorption spectrum of only DNTPD, andFIG. 23C shows the absorption spectrum of only molybdenum oxide. As isclear from the figures, the absorption spectrum of the complex materialcontaining DNTPD and molybdenum oxide has a peak which does not appearin the other absorption spectrums of only DNTPD and only molybdenumoxide. It is considered that this is because holes are generated byinteraction of DNTPD and molybdenum oxide.

Embodiment 4

This embodiment describes an example of controlling a light-emissionspectrum and viewing-angle dependence of light emission by changing thethickness of a hole-generating layer, which is a so-called opticaldesign of a light-emitting element with reference to FIGS. 24 and 25.

First, a method for manufacturing a light-emitting element in thisembodiment is described. The light-emitting element in this embodimentconforms to the structure of the light-emitting element shown inEmbodiment Mode 1. In this embodiment, a glass substrate is used as theinsulator 100. ITO containing silicon is formed over the glass substrateby a sputtering method, thereby forming the anode 101 in 110 nm thick.

Subsequently, the first hole-generating layer 102 is formed withmolybdenum oxide and α-NPD by co-evaporating molybdenum oxide and α-NPDover the anode 101. Here, the thickness of the first hole-generatinglayer 102 is set to 50 nm. The mass ratio between α-NPD and molybdenumoxide is set to 4:1.

Next, the light-emitting layer 104 is formed over the firsthole-generating layer 102. The light-emitting layer 104 has athree-layer structure including a hole-transporting layer, a layer inwhich a light-emitting material is diffused, and anelectron-transporting layer in order from the side of the firsthole-generating layer 102. The hole-transporting layer is formed withα-NPD in 10 nm thick by a vacuum evaporating method. The layer in whichthe light-emitting material is diffused is formed with Alq₃ and coumarin6 in 40 nm thick by a co-evaporating method. The electron-transportinglayer is formed with only Alq₃ in 10 nm thick by a vacuum evaporatingmethod. It is to be noted that the layer in which the light-emittingmaterial is diffused is adjusted so that the proportion between the Alq₃and coumarin 6 is 1:0.01 in mass ratio.

Subsequently, the electron-generating layer 105 is formed with Alq₃ andlithium in 10 nm thick by co-evaporating Alq₃ and lithium over thelight-emitting layer 104. The mass ratio between Alq₃ and lithium isadjusted so as to be 1:0.01.

Next, the second hole-generating layer 103 is formed with molybdenumoxide and α-NPD by co-evaporating molybdenum oxide and α-NPD over theelectron-generating layer 105. Further, the mass ratio between α-NPD andmolybdenum oxide is adjusted so as to be 2:1.

The cathode 106 is formed with aluminum over the second hole-generatinglayer 103. The film thickness is set to 100 nm.

In the light-emitting element having the above structure according tothe present invention, holes are injected from the secondhole-generating layer 103 to the second electrode by applying voltage.Further, electrons are injected from the electron-generating layer 105to the light-emitting layer 104. Moreover, holes are injected from thefirst hole-generating layer 102 to the light-emitting layer 104. In thelight-emitting layer, the injected holes and electrons are recombined,thereby providing light from coumarin 6.

In this embodiment, light is extracted from the light-emitting elementtoward the side of the glass substrate over which the light-emittingelement is formed, and the cathode 106 serves as a reflection electrode.Moreover, by changing the thickness of the second hole-generating layer103, the optical length of light returning after reflecting on thereflection electrode is adjusted. Accordingly, an interference statebetween the light emitted to the direction of the glass substrate afterreflecting on the reflection electrode and light directly emitted fromthe light-emitting element changes.

FIG. 24 is a graph showing the relation between the current efficiencyand the optical distance to the reflection electrode from the layer inwhich the light-emitting material is diffused, when the optical distanceis changed by changing the thickness of the second hole-generating layer103. Thus, it is to be understood that the emission efficiency changesperiodically by changing the optical distance to the reflectionelectrode from the layer in which the light-emitting material isdiffused. By adjusting the optical distance, it is possible to improveor suppress the emission efficiency.

FIG. 25 is a graph showing the change of a light-emission spectrum inthe case of changing the thickness of the second hole-generating layer103 between 140 nm and 280 nm. The thickness of the secondhole-generating layer 103 is 140 nm in an element 1, 160 nm in anelement 2, 180 nm in an element 3, 200 nm in an element 4, 220 nm in anelement 5, 240 nm in an element 6, 260 nm in an element 7, and 280 nm inan element 8. It is to be understood from the graph that the maximumwavelength and the spectrum shape of the light change when the opticaldistance to the reflection electrode from the layer in which thelight-emitting material is diffused is changed by changing the thicknessof the second hole-generating layer 103. Accordingly, it becomespossible to control the color or the color purity of light emitted fromthe light-emitting element by adjusting the optical distance.

This application is based on Japanese Patent Application serial No.2004-227734 filed in Japan Patent Office on Aug. 4, 2004, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An electronic device comprising: a light-emittingdevice comprising: a substrate comprising an organic compound; an anodeover the substrate; a first layer over the anode, the first layergenerating holes; a light emitting layer comprising a light emittingmaterial over the first layer; a second layer over the light emittinglayer, the second layer generating electrons; a third layer comprisingmetal oxide over the second layer, the third layer generating holes; anda cathode being in contact with the third layer.
 2. The electronicdevice according to claim 1, further comprising a counter substrate overthe cathode, wherein the substrate comprises a material selected fromthe group consisting of polyimide, acrylic, polyethylene terephthalate,polycarbonate, polyacrylate, and polyethersulfone.
 3. The electronicdevice according to claim 1, wherein the metal oxide is selected fromthe group consisting of vanadium oxide, molybdenum oxide, cobalt oxide,and nickel oxide.
 4. The electronic device according to claim 1, whereinthe light-emitting device is a display device.
 5. The electronic deviceaccording to claim 1, further comprising a counter substrate over thecathode, wherein the substrate is a plastic substrate.
 6. The electronicdevice according to claim 1, further comprising a counter substrate overthe cathode.
 7. The electronic device according to claim 1, wherein adisplay portion comprises the light-emitting device.
 8. An electronicdevice comprising: a light-emitting device comprising: a substratecomprising an organic compound; a semiconductor layer over thesubstrate; a gate electrode adjacent to the semiconductor layer with aninsulating layer interposed between the gate electrode and thesemiconductor layer; an anode over the substrate, the anode electricallyconnected to the semiconductor layer; a first layer over the anode, thefirst layer generating holes; a light emitting layer comprising a lightemitting material over the first layer; a second layer over the lightemitting layer, the second layer generating electrons; a third layercomprising metal oxide over the second layer, the third layer generatingholes; and a cathode being in contact with the third layer.
 9. Theelectronic device according to claim 8, further comprising a countersubstrate over the cathode, wherein the substrate comprises a materialselected from the group consisting of polyimide, acrylic, polyethyleneterephthalate, polycarbonate, polyacrylate, and polyethersulfone. 10.The electronic device according to claim 8, wherein the metal oxide isselected from the group consisting of vanadium oxide, molybdenum oxide,cobalt oxide, and nickel oxide.
 11. The electronic device according toclaim 8, wherein the semiconductor layer comprises one selected from agroup consisting of an amorphous semiconductor, a microcrystalsemiconductor, and crystalline semiconductor layer.
 12. The electronicdevice according to claim 8, wherein the light-emitting device is adisplay device.
 13. The electronic device according to claim 8, furthercomprising a counter substrate over the cathode, wherein the substrateis a plastic substrate.
 14. An electronic device comprising: alight-emitting device comprising: a substrate comprising an organiccompound; an anode over the substrate; a first layer over the anode, thefirst layer generating holes; a light emitting layer comprising a lightemitting material over the first layer; a second layer over the lightemitting layer, the second layer generating electrons; a third layercomprising metal oxide over the second layer, the third layer generatingholes; a cathode being in contact with the third layer; a countersubstrate over the cathode; and a resin filled between the cathode andthe counter substrate.
 15. The electronic device according to claim 14,wherein a display portion comprises the light-emitting device, andwherein the substrate comprises a material selected from the groupconsisting of polyimide, acrylic, polyethylene terephthalate,polycarbonate, polyacrylate, and polyethersulfone.
 16. The electronicdevice according to claim 14, wherein the metal oxide is selected fromthe group consisting of vanadium oxide, molybdenum oxide, cobalt oxide,and nickel oxide.
 17. The electronic device according to claim 14,wherein the resin comprises one selected from a group consisting ofpolyvinyl chloride, acrylic, polyimide, an epoxy resin, a silicon resin,polyvinyl butyral, and ethylene vinylene acetate.
 18. The electronicdevice according to claim 14, wherein the light-emitting device is adisplay device.
 19. The electronic device according to claim 14, whereina display portion comprises the light-emitting device, and wherein thesubstrate is a plastic substrate.